CTAB-assisted Hydrothermal Method for Tin Oxide Preparation as an Active Materials for Ethylene Gas Detection

Detection of ethylene gas, a chemical compound that affects the ripening rate of several kinds of fruits, can be one of the solutions to prevent the overripeness of fruits. Metal oxide semiconductors as an active material for gas sensors have several advantages, such as high sensitivity and low cost. Tin oxide is one of the metal oxide semiconductors that is often used to detect several kinds of gases. In this research, tin oxide will be used as an active material for detecting ethylene gas. Tin oxide was successfully synthesized by the CTAB-assisted hydrothermal method and characterized using XRD and SEM. The material is a tetragonal-phase tin oxide based on its diffraction peaks. Meanwhile, SEM imaging shows that tin oxide has morphologies of irregular nanoparticles. The performance test was done by exposing the materials to 20 ppm of ethylene gas in a temperature range of 150°C–350°C. From the test result, it was shown that at higher working temperatures, the sensitivity will be higher. At a working temperature of 350ºC, the sensitivity of ethylene detection is 4.9.


Introduction
Ethylene gas is referred to odorless and colorless phytohormone that has a crucial role in fruits ripening [1].Each fruit produces a different concentration of ethylene gas, from 0.1 ppm to 1000 ppm [1,2].Whereas its optimum ripening condition happens when the concentration of the ethylene gas is in the range from 10 ppm to 150 ppm [3].The increasing concentration of ethylene gas can cause damage to fruit physiology and can lead to fruit over-ripeness [2].Based on the role of ethylene gas during the ripeness process, fruits can be classified into climacteric fruits and non-climacteric fruits [4,5].Climacteric fruits, such as banana and mango, are types of fruits that will increase its respiration rate along with the increase of ethylene gas concentration [5,6].While non-climacteric fruits, such as grapes and watermelon, are types of fruits that will not be affected by the increase of ethylene gas concentration [5,7].The ripeness of fruits during its harvest time is the main determinant of its quality and storage time [8].The detection of ethylene gas can be a solution to prevent the over-ripeness of climacteric fruits due to the increasing of ethylene gas concentration.Nowadays, gas chromatography and optical detection are the most used methods to detect ethylene gas.Although those methods are expensive and can only be done in a laboratory [2].Chemiresistivebased gas sensor can be the simple detection methods, especially metal oxide semiconductors (MOS)based gas sensor [2,3,9].MOS-based gas sensors are popular gas detection methods due to its high sensitivity, high stability, potentially be miniaturized, and low cost [9][10][11].In this research, tin oxides that have been synthesized using CTAB-assisted hydrothermal were used for the detection of ethylene gas.The characterization and detection test results were analyzed to identify its performance for ethylene gas detection.

Tin Oxide Synthesis
Tin oxides were synthesized using hydrothermal methods [12].Initially, 5 ml of 0.3 M stannic chloride and 100 ml of 0.15 M sodium hydroxide were mixed and stirred vigorously to obtain a clear solution for 20 minutes.Afterwards, 2 mmol of cetyltrimethylammonium bromide (CTAB) powder was added to the above clear solution and then heated to completely dissolve the CTAB.The mixture was then poured into a 50 ml stainless steel autoclave lined with teflon and heated at 160°C for 12 h.After cooling to room temperature, the obtained white precipitate was collected by centrifugation, washed with ethanol several times, and then dried at 60ºC for several hours to obtain the SnO2 powder.

Characterization
The sample was characterized by X-ray diffraction with a Rigaku RINT 2500X diffractometer using monochromatic Cu-Kα radiation (λ = 1.5418Å) to identify the sample diffraction peaks.Field emission scanning electron microscope (FESEM) (Hitachi SU-3500) was used to examine the morphology and microstructure of the sample.

Sensor Fabrication and Gas Sensing Tests
Alumina substrates with silver electrodes were used for ethylene gas sensor testing.A certain amount of the sample was dispersed in ethanol to form a slurry.The slurry was then adhered to the electrode to form a thick film and dried at ambient temperature.A Picotest M3500A was used as a digital multimeter to measure changes in sensor resistance and recorded on a computer.An Omron G3PX-220EH device was used to control the temperature of the heaters used in the tests.The tests were done in a range of working temperatures from 150ºC to 350ºC.First, air flowed into the sample chamber.After the resistance of the sample are steady, ethylene flowed into the chamber and change into air again after 15 minutes.The sensitivity of materials in detecting ethylene is calculated by dividing the resistance of the sample in air by the resistance of the sample in ethylene (S=Ra/Rg).

Ethylene gas sensing test results
The gas sensing tests have been done in a range of working temperature from 150ºC to 350ºC using 20 ppm of pure ethylene gas.From the test results, the sensitivity was calculated by dividing resistance of sample in air (Ra) with the resistance of sample in gas target (Rg).Figure 2 shows the plot of response/sensitivity against working temperature.It shows that the sensitivity rises as the working temperature rise.The optimum working temperature of the SnO2 samples were not yet to be identify due to the limitations of the device that can only be done at the highest working temperature of 350ºC.

Figure 2. Plot response vs temperature in this research
The test result shows similar trend results to other work that use SnO2.Zhao et al has also done the same test using 10 ppm ethylene gas in a working temperature range of 50ºC to 400ºC [13].From the test results of the pure SnO2, the sensitivity is around 1 at relatively low working temperature (50ºC-200ºC), same as the test results that have been done in this research.It also shows that the sensitivity will start to rise exponentially at the working temperature of 250ºC, same as the results obtain in this research.1 shows the previous research on ethylene detection using binary metal oxides.Tin oxide has potential as an active material to detect ethylene and has been used several times for ethylene detection.It is difficult to compare with previous works due to differences in testing conditions, such as its working temperature and the ethylene concentration used in the testing.However, the tin oxide synthesized with CTAB-assisted hydrothermal has a high sensitivity among other materials used in previous work.This might be due to the high surface area with active sites for detecting ethylene.
With further development and investigation, this material could potentially be a high-performance ethylene sensor.

Conclusion
In summary, SnO2 has been successfully synthesized by CTAB-assisted hydrothermal.The result of the synthesis is a tetragonal phase of SnO2 with irregular nanoparticles.The result for the ethylene gas sensing test shows that SnO2 has higher performance at higher working temperature.The results obtained in this work were compared with other work and show similarity in response trends against work temperature.

Figure 1 .
(a) XRD patterns and (b) SEM imaging of SnO2 samples

Table 1
Previous research on binary metal oxides for ethylene detection